The use of this module in new code is discouraged. Other modules are available
which provide more straightforward and consistent interfaces. In particular,
XML::LibXML is highly recommended and XML::Twig is an excellent
alternative.

The major problems with this module are the large number of options (some of
which have unfortunate defaults) and the arbitrary ways in which these options
interact - often producing unexpected results.

Patches with bug fixes and documentation fixes are welcome, but new features
are unlikely to be added.

will slurp the configuration options into the hashref $config (because no
filename or XML string was passed as the first argument to XMLin() the name
and location of the XML file will be inferred from name and location of the
script). You can dump out the contents of the hashref using Data::Dumper:

Your script could then access the name of the log directory like this:

print $config->{logdir};

similarly, the second address on the server kalahari could be referenced as:

print $config->{server}->{kalahari}->{address}->[1];

Note: If the mapping between the output of Data::Dumper and the print
statements above is not obvious to you, then please refer to the references
tutorial (AKA: Marks very short tutorial about references) at perlreftut.

In this example, the ForceArray option was used to list elements that
might occur multiple times and should therefore be represented as arrayrefs
(even when only one element is present).

The KeyAttr option was used to indicate that each <server>
element has a unique identifier in the name attribute. This allows you
to index directly to a particular server record using the name as a hash key
(as shown above).

For simple requirements, thats really all there is to it. If you want to
store your XML in a different directory or file, or pass it in as a string or
even pass it in via some derivative of an IO::Handle, youll need to check out
OPTIONS. If you want to turn off or tweak the array folding feature (that
neat little transformation that produced $config->{server}) youll find options
for that as well.

If you want to generate XML (for example to write a modified version of
$config back out as XML), check out XMLout().

If your needs are not so simple, this may not be the module for you. In that
case, you might want to read WHERE TO FROM HERE?.

The XML::Simple module provides a simple API layer on top of an underlying XML
parsing module (either XML::Parser or one of the SAX2 parser modules). Two
functions are exported: XMLin() and XMLout(). Note: you can explicitly
request the lower case versions of the function names: xml_in() and
xml_out().

The simplest approach is to call these two functions directly, but an
optional object oriented interface (see OPTIONAL OO INTERFACE below)
allows them to be called as methods of an <B>XML::SimpleB> object. The object
interface can also be used at either end of a SAX pipeline.

Parses XML formatted data and returns a reference to a data structure which
contains the same information in a more readily accessible form. (Skip
down to EXAMPLES below, for more sample code).

XMLin() accepts an optional XML specifier followed by zero or more name =>
value option pairs. The XML specifier can be one of the following:

A filename

If the filename contains no directory components XMLin() will look for the
file in each directory in the SearchPath (see OPTIONS below) or in the
current directory if the SearchPath option is not defined. eg:

$ref = XMLin(/etc/params.xml);

Note, the filename - can be used to parse from STDIN.

undef

If there is no XML specifier, XMLin() will check the script directory and
each of the SearchPath directories for a file with the same name as the script
but with the extension .xml. Note: if you wish to specify options, you
must specify the value undef. eg:

$ref = XMLin(undef, ForceArray => 1);

A string of XML

A string containing XML (recognised by the presence of < and > characters)
will be parsed directly. eg:

Takes a data structure (generally a hashref) and returns an XMLencoding of
that structure. If the resulting XML is parsed using XMLin(), it should
return a data structure equivalent to the original (see caveats below).

The XMLout() function can also be used to output the XML as SAX events
see the Handler option and SAX SUPPORT for more details).

When translating hashes to XML, hash keys which have a leading - will be
silently skipped. This is the approved method for marking elements of a
data structure which should be ignored by XMLout. (Note: If these items
were not skipped the key names would be emitted as element or attribute names
with a leading - which would not be valid XML).

Some care is required in creating data structures which will be passed to
XMLout(). Hash keys from the data structure will be encoded as either XML
element names or attribute names. Therefore, you should use hash key names
which conform to the relatively strictXML naming rules:

Names in XML must begin with a letter. The remaining characters may be
letters, digits, hyphens (-), underscores (_) or full stops (.). It is also
allowable to include one colon (:) in an element name but this should only be
used when working with namespaces (<B>XML::SimpleB> can only usefully work with
namespaces when teamed with a SAX Parser).

You can use other punctuation characters in hash values (just not in hash
keys) however <B>XML::SimpleB> does not support dumping binary data.

If you break these rules, the current implementation of XMLout() will
simply emit non-compliant XML which will be rejected if you try to read it
back in. (A later version of <B>XML::SimpleB> might take a more proactive
approach).

Note also that although you can nest hashes and arrays to arbitrary levels,
circular data structures are not supported and will cause XMLout() to die.

If you wish to round-trip arbitrary data structures from Perl to XML and back
to Perl, then you should probably disable array folding (using the KeyAttr
option) both with XMLout() and with XMLin(). If you still dont get the
expected results, you may prefer to use XML::Dumper which is designed for
exactly that purpose.

<B>XML::SimpleB> supports a number of options (in fact as each release of
<B>XML::SimpleB> adds more options, the modules claim to the name Simple
becomes increasingly tenuous). If you find yourself repeatedly having to
specify the same options, you might like to investigate OPTIONAL OO
INTERFACE below.

If you cant be bothered reading the documentation, refer to
STRICT MODE to automatically catch common mistakes.

Because there are so many options, its hard for new users to know which ones
are important, so here are the two you really need to know about:

o

check out ForceArray because youll almost certainly want to turn it on

o

make sure you know what the KeyAttr option does and what its default value is
because it may surprise you otherwise (note in particular that KeyAttr
affects both XMLin and XMLout)

The option name headings below have a trailing comment - a hash followed by
two pieces of metadata:

o

Options are marked with in if they are recognised by XMLin() and
out if they are recognised by XMLout().

o

Each option is also flagged to indicate whether it is:

important - dont use the module until you understand this one
handy - you can skip this on the first time through
advanced - you can skip this on the second time through
SAX only - dont worry about this unless youre using SAX (or
alternatively if you need this, you also need SAX)
seldom used - youll probably never use this unless you were the
person that requested the feature

The options are listed alphabetically:

Note: option names are no longer case sensitive so you can use the mixed case
versions shown here; all lower case as required by versions 2.03 and earlier;
or you can add underscores between the words (eg: key_attr).

Because loading the <B>XML::ParserB> module and parsing an XML file can consume a
significant number of CPU cycles, it is often desirable to cache the output of
XMLin() for later reuse.

When parsing from a named file, <B>XML::SimpleB> supports a number of caching
schemes. The Cache option may be used to specify one or more schemes (using
an anonymous array). Each scheme will be tried in turn in the hope of finding
a cached pre-parsed representation of the XML file. If no cached copy is
found, the file will be parsed and the first cache scheme in the list will be
used to save a copy of the results. The following cache schemes have been
implemented:

storable

Utilises <B>Storable.pmB> to read/write a cache file with the same name as the
XML file but with the extension .stor

memshare

When a file is first parsed, a copy of the resulting data structure is retained
in memory in the <B>XML::SimpleB> modules namespace. Subsequent calls to parse
the same file will return a reference to this structure. This cached version
will persist only for the life of the Perl interpreter (which in the case of
mod_perl for example, may be some significant time).

Because each caller receives a reference to the same data structure, a change
made by one caller will be visible to all. For this reason, the reference
returned should be treated as read-only.

memcopy

This scheme works identically to memshare (above) except that each caller
receives a reference to a new data structure which is a copy of the cached
version. Copying the data structure will add a little processing overhead,
therefore this scheme should only be used where the caller intends to modify
the data structure (or wishes to protect itself from others who might). This
scheme uses <B>Storable.pmB> to perform the copy.

Warning! The memory-based caching schemes compare the timestamp on the file to
the time when it was last parsed. If the file is stored on an NFS filesystem
(or other network share) and the clock on the file server is not exactly
synchronised with the clock where your script is run, updates to the source XML
file may appear to be ignored.

When you use an <B>XML::SimpleB> object as a SAX handler, it will return a
simple tree data structure in the same format as XMLin() would return. If
this option is set (to a subroutine reference), then when the tree is built the
subroutine will be called and passed two arguments: a reference to the
<B>XML::SimpleB> object and a reference to the data tree. The return value from
the subroutine will be returned to the SAX driver. (See SAX SUPPORT for
more details).

This option should be set to 1 to force nested elements to be represented
as arrays even when there is only one. Eg, with ForceArray enabled, this
XML:

<opt>
<name>value</name>
</opt>

would parse to this:

{
name => [
value
]
}

instead of this (the default):

{
name => value
}

This option is especially useful if the data structure is likely to be written
back out as XML and the default behaviour of rolling single nested elements up
into attributes is not desirable.

If you are using the array folding feature, you should almost certainly enable
this option. If you do not, single nested elements will not be parsed to
arrays and therefore will not be candidates for folding to a hash. (Given that
the default value of KeyAttr enables array folding, the default value of this
option should probably also have been enabled too - sorry).

This alternative (and preferred) form of the ForceArray option allows you to
specify a list of element names which should always be forced into an array
representation, rather than the all or nothing approach above.

It is also possible (since version 2.05) to include compiled regular
expressions in the list - any element names which match the pattern will be
forced to arrays. If the list contains only a single regex, then it is not
necessary to enclose it in an arrayref. Eg:

When XMLin() parses elements which have text content as well as attributes,
the text content must be represented as a hash value rather than a simple
scalar. This option allows you to force text content to always parse to
a hash value even when there are no attributes. So for example:

The grouping element (<searchpath> in the example) must not contain any
attributes or elements other than the grouped element.

You can specify multiple grouping element to grouped element mappings in
the same hashref. If this option is combined with KeyAttr, the array
folding will occur first and then the grouped element names will be eliminated.

XMLout will also use the grouptag mappings to re-introduce the tags around
the grouped elements. Beware though that this will occur in all places that
the grouping tag name occurs - you probably dont want to use the same name
for elements as well as attributes.

Use the Handler option to have XMLout() generate SAX events rather than
returning a string of XML. For more details see SAX SUPPORT below.

Note: the current implementation of this option generates a string of XML
and uses a SAX parser to translate it into SAX events. The normal encoding
rules apply here - your data must be UTF8 encoded unless you specify an
alternative encoding via the XMLDecl option; and by the time the data reaches
the handler object, it will be in UTF8 form regardless of the encoding you
supply. A future implementation of this option may generate the events
directly.

In its attempt to return a data structure free of superfluous detail and
unnecessary levels of indirection, XMLin() normally discards the root
element name. Setting the KeepRoot option to 1 will cause the root element
name to be retained. So after executing this code:

$config = XMLin(<config tempdir="/tmp" />, KeepRoot => 1)

Youll be able to reference the tempdir as
$config->{config}->{tempdir} instead of the default
$config->{tempdir}.

Similarly, setting the KeepRoot option to 1 will tell XMLout() that the
data structure already contains a root element name and it is not necessary to
add another.

The key attribute names should be supplied in an arrayref if there is more
than one. XMLin() will attempt to match attribute names in the order
supplied. XMLout() will use the first attribute name supplied when
unfolding a hash into an array.

Note 1: The default value for KeyAttr is [name, key, id]. If you do
not want folding on input or unfolding on output you must set this option
to an empty list to disable the feature.

Note 2: If you wish to use this option, you should also enable the
ForceArray option. Without ForceArray, a single nested element will be
rolled up into a scalar rather than an array and therefore will not be folded
(since only arrays get folded).

This alternative (and preferred) method of specifying the key attributes
allows more fine grained control over which elements are folded and on which
attributes. For example the option KeyAttr => { package => id } will cause
any package elements to be folded on the id attribute. No other elements
which have an id attribute will be folded at all.

Note: XMLin() will generate a warning (or a fatal error in STRICT MODE)
if this syntax is used and an element which does not have the specified key
attribute is encountered (eg: a package element without an id attribute, to
use the example above). Warnings can be suppressed with the lexical
no warnings; pragma or no warnings XML::Simple;.

Two further variations are made possible by prefixing a + or a - character
to the attribute name:

By default, XMLout() will translate the characters <, >, & and
" to <, >, & and &quot respectively. Use this option to
suppress escaping (presumably because youve already escaped the data in some
more sophisticated manner).

Set this option to 1 to disable XMLout()s default pretty printing mode.
With this option enabled, the XML output will all be on one line (unless there
are newlines in the data) - this may be easier for downstream processing.

* Actually, sorting is alphabetical but key attribute or element names (as in
KeyAttr) sort first. Also, when a hash of hashes is unfolded, the elements
are sorted alphabetically by the value of the key field.

This option controls namespace expansion - the translation of element and
attribute names of the form prefix:name to {uri}name. For example the
element name xsl:template might be expanded to:
{http://www.w3.org/1999/XSL/Transform}template.

By default, XMLin() will return element names and attribute names exactly as
they appear in the XML. Setting this option to 1 will cause all element and
attribute names to be expanded to include their namespace prefix.

Note: You must be using a SAX parser for this option to work (ie: it does not
work with XML::Parser).

This option also controls whether XMLout() performs the reverse translation
from {uri}name back to prefix:name. The default is no translation. If
your data contains expanded names, you should set this option to 1 otherwise
XMLout will emit XML which is not well formed.

Note: You must have the XML::NamespaceSupport module installed if you want
XMLout() to translate URIs back to prefixes.

Note: This option is now officially deprecated. If you find it useful, email
the author with an example of what you use it for. Do not use this option to
set the ProtocolEncoding, thats just plain wrong - fix the XML.

This option allows you to pass parameters to the constructor of the underlying
XML::Parser object (which of course assumes youre not using SAX).

By default, when XMLout() generates XML, the root element will be named
opt. This option allows you to specify an alternative name.

Specifying either undef or the empty string for the RootName option will
produce XML with no root elements. In most cases the resulting XML fragment
will not be well formed and therefore could not be read back in by XMLin().
Nevertheless, the option has been found to be useful in certain circumstances.

If you pass XMLin() a filename, but the filename include no directory
component, you can use this option to specify which directories should be
searched to locate the file. You might use this option to search first in the
users home directory, then in a global directory such as /etc.

If a filename is provided to XMLin() but SearchPath is not defined, the
file is assumed to be in the current directory.

If the first parameter to XMLin() is undefined, the default SearchPath
will contain only the directory in which the script itself is located.
Otherwise the default SearchPath will be empty.

This option controls what XMLin() should do with empty elements (no
attributes and no content). The default behaviour is to represent them as
empty hashes. Setting this option to a true value (eg: 1) will cause empty
elements to be skipped altogether. Setting the option to undef or the empty
string will cause empty elements to be represented as the undefined value or
the empty string respectively. The latter two alternatives are a little
easier to test for in your code than a hash with no keys.

The option also controls what XMLout() does with undefined values. Setting
the option to undef causes undefined values to be output as empty elements
(rather than empty attributes), it also suppresses the generation of warnings
about undefined values. Setting the option to a true value (eg: 1) causes
undefined values to be skipped altogether on output.

This option allows variables in the XML to be expanded when the file is read.
(there is no facility for putting the variable names back if you regenerate
XML using XMLout).

A variable is any text of the form ${name} which occurs in an attribute
value or in the text content of an element. If name matches a key in the
supplied hashref, ${name} will be replaced with the corresponding value from
the hashref. If no matching key is found, the variable will not be replaced.
Names must match the regex: [\w.]+ (ie: only word characters and dots are
allowed).

In addition to the variables defined using Variables, this option allows
variables to be defined in the XML. A variable definition consists of an
element with an attribute called attr_name (the value of the VarAttr
option). The value of the attribute will be used as the variable name and the
text content of the element will be used as the value. A variable defined in
this way will override a variable defined using the Variables option. For
example:

The procedural interface is both simple and convenient however there are a
couple of reasons why you might prefer to use the object oriented (OO)
interface:

o

to define a set of default values which should be used on all subsequent calls
to XMLin() or XMLout()

o

to override methods in <B>XML::SimpleB> to provide customised behaviour

The default values for the options described above are unlikely to suit
everyone. The OOinterface allows you to effectively override <B>XML::SimpleB>s
defaults with your preferred values. It works like this:

First create an XML::Simple parser object with your preferred defaults:

my $xs = XML::Simple->new(ForceArray => 1, KeepRoot => 1);

then call XMLin() or XMLout() as a method of that object:

my $ref = $xs->XMLin($xml);
my $xml = $xs->XMLout($ref);

You can also specify options when you make the method calls and these values
will be merged with the values specified when the object was created. Values
specified in a method call take precedence.

Note: when called as methods, the XMLin() and XMLout() routines may be
called as xml_in() or xml_out(). The method names are aliased so the
only difference is the aesthetics.

You can make your own class which inherits from XML::Simple and overrides
certain behaviours. The following methods may provide useful hooks upon
which to hang your modified behaviour. You may find other undocumented methods
by examining the source, but those may be subject to change in future releases.

handle_options(direction, name => value ...)

This method will be called when one of the parsing methods or the XMLout()
method is called. The initial argument will be a string (either in or out)
and the remaining arguments will be name value pairs.

default_config_file()

Calculates and returns the name of the file which should be parsed if no
filename is passed to XMLin() (default: $0.xml).

build_simple_tree(filename, string)

Called from XMLin() or any of the parsing methods. Takes either a file name
as the first argument or undef followed by a string as the second
argument. Returns a simple tree data structure. You could override this
method to apply your own transformations before the data structure is returned
to the caller.

new_hashref()

When the simple tree data structure is being built, this method will be
called to create any required anonymous hashrefs.

sorted_keys(name, hashref)

Called when XMLout() is translating a hashref to XML. This routine returns
a list of hash keys in the order that the corresponding attributes/elements
should appear in the output.

escape_value(string)

Called from XMLout(), takes a string and returns a copy of the string with
XML character escaping rules applied.

escape_attr(string)

Called from XMLout(), to handle attribute values. By default, just calls
escape_value(), but you can override this method if you want attributes
escaped differently than text content.

numeric_escape(string)

Called from escape_value(), to handle non-ASCII characters (depending on the
value of the NumericEscape option).

copy_hash(hashref, extra_key => value, ...)

Called from XMLout(), when unfolding a hash of hashes into an array of
hashes. You might wish to override this method if youre using tied hashes and
dont want them to get untied.

XML::Simple implements three caching schemes (storable, memshare and
memcopy). You can implement a custom caching scheme by implementing
two methods - one for reading from the cache and one for writing to it.

For example, you might implement a new dbm scheme that stores cached data
structures using the MLDBM module. First, you would add a
cache_read_dbm() method which accepted a filename for use as a lookup key
and returned a data structure on success, or undef on failure. Then, you would
implement a cache_read_dbm() method which accepted a data structure and a
filename.

the following common mistakes will be detected and treated as fatal errors

o

Failing to explicitly set the KeyAttr option - if you cant be bothered
reading about this option, turn it off with: KeyAttr => [ ]

o

Failing to explicitly set the ForceArray option - if you cant be bothered
reading about this option, set it to the safest mode with: ForceArray => 1

o

Setting ForceArray to an array, but failing to list all the elements from the
KeyAttr hash.

o

Data error - KeyAttr is set to say { part => partnum } but the XML contains
one or more <part> elements without a partnum attribute (or nested
element). Note: ifstrict mode is not set but use warnings; is in force,
this condition triggers a warning.

o

Data error - as above, but non-unique values are present in the key attribute
(eg: more than one <part> element with the same partnum). This will
also trigger a warning ifstrict mode is not enabled.

o

Data error - as above, but value of key attribute (eg: partnum) is not a
scalar string (due to nested elements etc). This will also trigger a warning
ifstrict mode is not enabled.

From version 1.08_01, <B>XML::SimpleB> includes support for SAX (the Simple API
for XML) - specifically SAX2.

In a typical SAX application, an XML parser (or SAX driver) module generates
SAX events (start of element, character data, end of element, etc) as it parses
an XML document and a handler module processes the events to extract the
required data. This simple model allows for some interesting and powerful
possibilities:

o

Applications written to the SAX API can extract data from huge XML documents
without the memory overheads of a DOM or tree API.

o

The SAX API allows for plug and play interchange of parser modules without
having to change your code to fit a new modules API. A number of SAX parsers
are available with capabilities ranging from extreme portability to blazing
performance.

o

A SAX filter module can implement both a handler interface for receiving
data and a generator interface for passing modified data on to a downstream
handler. Filters can be chained together in pipelines.

o

One filter module might split a data stream to direct data to two or more
downstream handlers.

o

Generating SAX events is not the exclusive preserve of XML parsing modules.
For example, a module might extract data from a relational database using DBI
and pass it on to a SAX pipeline for filtering and formatting.

<B>XML::SimpleB> can operate at either end of a SAX pipeline. For example,
you can take a data structure in the form of a hashref and pass it into a
SAX pipeline using the Handler option on XMLout():

You can build a filter by using an XML::Simple object as a handler and setting
its DataHandler option to point to a routine which takes the resulting tree,
modifies it and sends it off as SAX events to a downstream handler:

If you dont care which parser module <B>XML::SimpleB> uses then skip this
section entirely (it looks more complicated than it really is).

<B>XML::SimpleB> will default to using a <B>SAXB> parser if one is available or
<B>XML::ParserB> ifSAX is not available.

You can dictate which parser module is used by setting either the environment
variable XML_SIMPLE_PREFERRED_PARSER or the package variable
$XML::Simple::PREFERRED_PARSER to contain the module name. The following rules
are used:

o

The package variable takes precedence over the environment variable if both are defined. To force <B>XML::SimpleB> to ignore the environment settings and use
its default rules, you can set the package variable to an empty string.

If the preferred parser is set to some other value, then it is assumed to be
the name of a SAX parser module and is passed to XML::SAX::ParserFactory.
If XML::SAX is not installed, or the requested parser module is not
installed, then XMLin() will die.

o

If the preferred parser is not defined at all (the normal default
state), an attempt will be made to load XML::SAX. If XML::SAX is
installed, then a parser module will be selected according to
XML::SAX::ParserFactorys normal rules (which typically means the last SAX
parser installed).

Note: The <B>XML::SAXB> distribution includes an XML parser written entirely in
Perl. It is very portable but it is not very fast. You should consider
installing XML::LibXML or XML::SAX::Expat if they are available for your
platform.

The XML standard is very clear on the issue of non-compliant documents. An
error in parsing any single element (for example a missing end tag) must cause
the whole document to be rejected. <B>XML::SimpleB> will die with an appropriate
message if it encounters a parsing error.

If dying is not appropriate for your application, you should arrange to call
XMLin() in an eval block and look for errors in $@. eg:

Note, there is a common misconception that use of <B>evalB> will significantly
slow down a script. While that may be true when the code being evald is in a
string, it is not true of code like the sample above.

Anonymous arrays can be nested to arbitrary levels and as a special case, if
the surrounding tags for an XML document contain only an anonymous array the
arrayref will be returned directly rather than the usual hashref:

Elements which only contain text content will simply be represented as a
scalar. Where an element has both attributes and text content, the element
will be represented as a hashref with the text content in the content key
(see the ContentKey option):

Mixed content (elements which contain both text content and nested elements)
will be not be represented in a useful way - element order and significant
whitespace will be lost. If you need to work with mixed content, then
XML::Simple is not the right tool for your job - check out the next section.

You dont mind that when things get slurped into a hash the order is lost

o

You dont want fine-grained control of the formatting of generated XML

o

You would never use a hash key that was not a legal XML element name

o

You dont need help converting between different encodings

In a serious XML project, youll probably outgrow these assumptions fairly
quickly. This section of the document used to offer some advice on choosing a
more powerful option. That advice has now grown into the Perl-XML FAQ
document which you can find at: <http://perl-xml.sourceforge.net/faq/>

The advice in the FAQ boils down to a quick explanation of tree versus
event based parsers and then recommends:

For event based parsing, use SAX (do not set out to write any new code for
XML::Parsers handler API - it is obsolete).

For tree-based parsing, you could choose between the Perlish approach of
XML::Twig and more standards based DOM implementations - preferably one with
XPath support such as XML::LibXML.